PS: Proper AST support for switch arguments

[MathiasVP]

Previously, we did not handle switch arguments when constructing the AST.

This PR fixes this by synthesizing a Boolean $true argument so that a switch argument such as Foo -MyFlag looks the same (from the AST point-of-view) as Foo -MyFlag:$true.

Thanks for reporting this @chanel-y! 🙇

Commit-by-commit review recommended.